This invention relates to a drive belt tension control apparatus for an inverted router. An inverted router is a particular type of woodworking machine wherein the operator is protected from the router bit by positioning the workpiece on a table and making cut in the workpiece from the underside. The table has on opening through which protrudes the router bit, which is mounted on a spindle. A foot operated pedal controls movement of the router bit upwardly through the table opening into workpiece engagement and downwardly out of workpiece engagement. The router bit moves vertically on a precise axis in order that the cuts be precisely positioned. This axis is defined by a guide pin which is positioned on the same axis and is used to guide the workpiece on the table.
In most types of inverted routers the spindle is driven by a motor through a pulley and belt arrangement. It is important to maintain the tension on this belt within a relatively narrow range. If the belt tension is too high the spindle may be deflected off of the proper rotational axis and the life of the bearings in the spindle assembly substantially shortened. If the tension is too low slippage between the belt and one or both of the pulleys can reduce the speed of the spindle to below its optimum speed, especially when the router's spindle is under load while a cut is being made. Reduced spindle speed can cause the router bit to chatter in the workpiece, stop, and cause excessive wear to the belt and pulley.
Prior art belt tensioning devices comprise brackets which are loosened while the tension is adjusted, and then tightened to maintain the tension at that point. However, this method is highly inaccurate and time-consuming. Even if the tension is adjusted properly, belt elongation, temperature, humidity and vibration can very quickly cause the tension to move out of proper adjustment. Ideally, tension should be controlled constantly, and should be variable, if necessary, to take into account environmental and operating variations. Using the motor as a tensioning weight cannot be done since the motor and spindle must be maintained in proper axial alignment with each other.